1. Field of the Invention
This invention concerns a chemical mechanical polishing slurry including a complexing agent, at least one oxidizer, and at least one abrasive. The slurry is notable in that it does not include a film-forming agent. The amounts and types of oxidizers and complexing agents are selected to maximize polishing while minimizing the depth of any passivation layer on the substrate surface. The chemical mechanical polishing slurry of this invention is useful for polishing metal layers and thin-films associated with semiconductor manufacturing. More particularly this invention concerns three component chemical mechanical polishing slurries that are especially formulated to polish multiple metal layers and thin-films where one of the layers or films is comprised of copper or a copper containing alloy.
2. Description of the Art
Integrated circuits are made up of millions of active devices formed in or on a silicon substrate. The active devices, which are initially isolated from one another, are interconnected to form functional circuits and components. The devices are interconnected through the use of well-known multilevel interconnections. Interconnection structures normally have a first layer of metallization, an interconnection layer, a second level of metallization, and sometimes a third and subsequent level of metallization. Interlevel dielectrics, such as doped and undoped silicon dioxide (SiO2), are used to electrically isolate the different levels of metallization in a silicon substrate or well. The electrical connections between different interconnection levels are made through the use of metallized vias. U.S. Pat. No. 4,789,648, which is incorporated herein by reference, describes a method for preparing multiple metallized layers and metallized vias in insulator films. In a similar manner, metal contacts are used to form electrical connections between interconnection levels and devices formed in a well. The metal vias and contacts may be filled with various metals and alloys including titanium (Ti), titanium nitride (TiN), tantalum (Ta), aluminum copper (Al—Cu), aluminum silicon (Al—Si), copper (Cu), tungsten (W), and combinations thereof. The metal vias and contacts generally employ an adhesion layer such as titanium nitride (TiN) and/or titanium (Ti) to adhere the metal layer to the SiO2 substrate. At the contact level, the adhesion layer acts as a diffusion barrier to prevent the filled metal and SiO2 from reacting.
In one semiconductor manufacturing process, metallized vias or contacts are formed by a blanket metal deposition followed by a chemical mechanical polish (CMP) step. In a typical process, via holes are etched through an interlevel dielectric (ILD) to interconnection lines or to a semiconductor substrate. Next, a thin adhesion layer such as titanium nitride and/or titanium is generally formed over the ILD and is directed into the etched via hole. Then, a metal film is blanket deposited over the adhesion layer and into the via hole. Deposition is continued until the via hole is filled with the blanket deposited metal. Finally, the excess metal is removed by chemical mechanical polishing, (CMP) to form metal vias. Processes for manufacturing and/or CMP of vias are disclosed in U.S. Pat. Nos. 4,671,851, 4,910,155 and 4,944,836.
In a typical chemical mechanical polishing process, the substrate is placed in direct contact with a rotating polishing pad. A carrier applies pressure against the backside of the substrate. During the polishing process, the pad and table are rotated while a downward force is maintained against the substrate back. An abrasive and chemically reactive solution, commonly referred to as a “slurry,” is applied to the pad during polishing. The slurry initiates the polishing process by chemically reacting with the film being polished. The polishing process is facilitated by the rotational movement of the pad relative to the substrate as slurry is provided to the wafer/pad interface. Polishing is continued in this manner until the desired film on the insulator is removed. The slurry composition is an important factor in the CMP step. Depending on the choice of the oxidizing agent, the abrasive, and other useful additives, the polishing slurry can be tailored to provide effective polishing to metal layers at desired polishing rates while minimizing surface imperfections, defects, corrosion and erosion. Furthermore, the polishing slurry may be used to provide controlled polishing selectivities to other thin-film materials used in current integrated circuit technology such as titanium, titanium nitride and the like.
Typically, CMP polishing slurries contain an abrasive material, such as silica or alumina, suspended in an oxidizing, aqueous medium. For example, U.S. Pat. No. 5,244,534 to Yu et al. reports a slurry containing alumina, hydrogen peroxide, and either potassium or ammonium hydroxide that is useful to remove tungsten at predictable rates with little removal of the underlying insulating layer. U.S. Pat. No. 5,209,816 to Yu et al. discloses a slurry comprising perchloric acid, hydrogen peroxide and a solid abrasive material in an aqueous medium that is useful for polishing aluminum. U.S. Pat. No. 5,340,370 to Cadien et al. discloses a tungsten polishing slurry comprising approximately 0.1M potassium ferricyanide, approximately 5 wt. % silica and potassium acetate. Acetic acid is added to buffer the pH at approximately 3.5.
U.S. Pat. No. 4,789,648 to Beyer et al. discloses a slurry formulation using alumina abrasives in conjunction with sulfuric, nitric, and acetic acids and deionized water. U.S. Pat. Nos. 5,391,258 and 5,476,606 disclose slurries for polishing a composite of metal and silica which includes an aqueous medium, abrasive particles and an anion which controls the rate of silica removal. Other polishing slurries for use in CMP applications are described in U.S. Pat. No. 5,527,423 to Neville et al., U.S. Pat. No. 5,354,490 to Yu et al., U.S. Pat. No. 5,157,876 to Medellin, U.S. Pat. No. 5,137,544 to Medellin, and U.S. Pat. No. 4,956,313 to Cote et al.
There are various mechanisms disclosed in the prior art by which metal surfaces can be polished with slurries. The metal surface may be polished using a slurry in which a surface film is not formed in which case the process proceeds by mechanical removal of metal particles and their dissolution in the slurry. In such a mechanism, the chemical dissolution rate should be slow in order to avoid wet etching. A more preferred mechanism is, however, one where a thin abradable layer is continuously formed by reaction between the metal surface and one or more components in the slurry such as a complexing agent and/or a film-forming layer. The thin abradable layer is then removed in a controlled manner by mechanical action. Once the mechanical polishing process has stopped a thin passive film remains on the surface and controls the wet etching process. Controlling the chemical mechanical polishing process is much easier when a CMP slurry polishes using this mechanism.
Efforts to develop copper CMP slurries are disclosed in the literature. The RPI effort (J. M. Steigerwald et al, Electrochemical Potential Measurements during the Chemical-Mechanical Polishing of Copper Thin Films, Mat. Res. Soc. Symp. 337, 133 (1994)) is focused on the use of ammonium compounds (ammonium nitrate, chloride, hydroxide), nitric acid, and alumina abrasive. Copper dissolution of 2 nm/min (as measured electrochemically) is assumed to proceed from a film-free surface. Polishing rates, however, are reported to be in excess of 400 nm/min. The discrepancy is explained by importance given to the mechanical action, forming Cu debris, which is then dissolved by solution. Selectivity factors are not given.
Q. Luo et al, Chemical-Mechanical Polishing of Copper in Acidic Media, Proceedings—First International Chemical-Mechanical Polish (CMP) for VLSI/LSI Multilevel Interconnection Conference (CMP-MIC), Santa Barbara, Feb. 22-23, 1996 discloses using a CMP slurry including a very aggressive etchant, Fe-nitrate, pH 1-2, in combination with an inhibitor (benzotriazole), a slurry stabilizing surfactant (poly-ethylene-glycol) and alumina. The chemical reaction is apparently controlled by a formation of a corrosion inhibiting film, namely Cu-BTA, with surfactant undermining its protectiveness. Selectivity to oxide is given as 15:1 to 45:1.
CMP electrochemical work at Sematech laboratories is disclosed in R. Carpio et al, Initial Study On Copper CMP Slurry Chemistries, Thin Solid Films, 262 (1995). The reference explores the use of electrochemistry in the fundamental characterization of plausible slurries. In addition to several others, potassium permanganate is used as a slurry oxidizer.
H. Hirabayashi et al, Chemical Mechanical Polishing of Copper Using A Slurry Composed of Glycine and Hydrogen Peroxide, Proceedings—First International Chemical-Mechanical Polish (CMP) for VLSI/LSI Multilevel Interconnection Conference (CMP-MIC), Santa Barbara, Feb. 22-23 1996, and Japanese Kokai Patent Application No. 8 (1996) 83780 disclose a mixture of glycine, hydrogen peroxide and silica, with or without benzotriazole, for the CMP process of Cu with a low corrosion rate and defect level. The references disclose that CMP slurries incorporating a chemical agent, such as benzotriazole and n-benzoyl-n-phenylhydroxylamine, form a protective film on copper. The removal rate varies, depending on the concentration of slurry components. An optimized rate of 120 nm/min was reported, with TiN rate of 30 nm/min and dishing of 200 nm across the 15 μm wide structures.
In the course of polishing substrates using slurries including passivating agents such as benzotriazole (BTA), it has been discovered that reaction of BTA with the copper surface during polishing produces a surface film which is very resistant to the mechanical action of the abrasive, making removal of the surface film difficult. Furthermore, the degree of passivation is time dependent and not easy to control making automation of substrate polishing processes difficult. In addition, BTA undergoes oxidative degradation. Thus, a slurry containing BTA and an oxidizer has a short useful pot life that limits manufacturing usefulness. These properties of passivating agents, such as BTA, make it difficult to use machines to polish passivated substrates in a reproducible manner.
Despite the desirability of using passivating agents in a film forming mechanism in CMP process, there remains problems with formulating CMP slurries that are able to control the thickness of the passivating layer of film formed as well as problems ensuring that the film formed is abradable. These problems can result in film forming CMP slurries that exhibit unacceptably low polishing rates or poor polishing results. Thus, a need remains for a CMP slurry that is capable of forming a removable thin passive layer on a substrate surface, and more particularly on the surface of a copper alloy containing substrate. A desirable CMP slurry will exhibit good thin film polishing selectivities and simultaneously give polished substrates with minimal dishing and low defectivity.